[lib] Fix determinism bug in the optimal parser

[lib] Fix determinism bug in the optimal parser

`ZSTD_insertBt1()` has a speed optimization that skips the prefix of
very long matches.

https://github.com/facebook/zstd/blob/40def70387f99b239f3f566ba399275a8fd44cde/lib/compress/zstd_opt.c#L476

This optimization is based off the length longest match found. However,
when indices are reset, we only ensure that we can reference the whole
window starting from `ip`. If the previous block ended with a long match
then `nextToUpdate` could be much less than `ip`. It might be far enough
back that `nextToUpdate < maxDist`, so it doesn't have a full window of
data to reference. This can cause non-determinism bugs, because we may
find a match that is beyond `ip - maxDist`, and may sometimes be
un-referencable, and that match triggers the speed optimization.

The fix is to base the `windowLow` off of the `target` of
`ZSTD_updateTree_internal()`, because anything below that value will be
obsolete by the time `ZSTD_updateTree_internal()` completes.